Why we need assumptions while we study the behaviour of a gas

 For any solid object its motion can be described well with the help of Newton's laws of motion

Similarly a gas enclosed in a container can be characterized by macroscopic state variable like pressure , volume and temperature

However, as gas molecule are always in random motion it is difficult to understand behaviour of a gas in terms of motion of a single molecule

The number of molecule in the gas is so large (=10²³/ m³ ) that motion of individual molecule cannot be related with macroscopic parameters P,V,T and energy E

Hence, it is necessary to make certain assumptions while studying behaviour of a gas.